The modernization and urbanization of developing countries places an increasing demand on supplies of fossil fuels and the use of such fuels places an increasing burden on the environment. As market demand drives fuel prices upward and as increased consumption accelerates environmental pollution, alternative energy sources become more economically feasible and socially popular. Among the various alternative energy sources, solar energy is one of the most promising due to the endless supply of free energy from the sun. One method of harnessing the sun's energy is through optical solar concentration.
Solar concentration is used in combination with traditional photovoltaic cells to reduce the area of cells necessary to generate a given amount of electrical energy. In particular, sunlight shining on a solar concentrator is optically concentrated and transmitted to a solar cell. Through optical concentration, or geometric gain, a smaller photovoltaic cell can be used to generate a given amount of electrical energy. By reducing the photovoltaic cell area necessary to generate a given amount of electrical energy, optical concentration reduces the cost of energy production.
There are two distinct approaches to solar concentration. One approach uses lenses or mirrors and tracks the sun throughout the day. This tracking approach can produce very high concentration (e.g. greater than 500 suns) but requires tracking to within 0.1 degree and, therefore, is expensive and susceptible to tracking errors that may reduce performance. Another approach does not track the sun. One example of this non-tracking approach uses fixed lenses and mirrors, which produces relatively low concentration (e.g. less than 5 suns). Another example is the luminescent solar concentrator (LSC).
Optical solar concentration provides a realistic, near-term prospect for leveraging the cost and expanding the generation capacity of today's established solar cell technologies. The maximum concentration ratio (CR) obtainable using linear geometric optical systems involving lenses, mirrors, or diffractive optics, is fundamentally limited by the acceptance angle (θacc) of the system and the refractive index (nout) at its output aperture through the well-known sine law, CR≦(nout/sin θacc)2. Maintaining high concentration (CR>100) throughout the day thus demands that these concentrators track the sun with high precision, which drives up both capital and maintenance costs of the overall system.
LSCs were developed in the 1970s and have a high fundamental concentration (e.g. greater than 100 suns). LSCs were introduced as an alternative, non-tracking approach that preserves, at least in principle, the potential for high concentration. However, technical issues have limited the utility of LSCs to date. LSCs provide a simple means to concentrate sunlight without tracking the sun. These devices operate by absorbing light and then re-emitting it at lower frequency, typically into the confined modes of a transparent slab, where it is transported toward photovoltaic cells attached to the edges. In the thermodynamic limit, concentration ratio exceeding the equivalent of 100 suns is possible, however, in actual LSCs, optical propagation loss due mostly to reabsorption limits the concentration ratio to approximately 10.
In contrast to their ‘passive’ geometric optical counterparts, LSCs actively shift the optical frequency by absorbing sunlight and re-emitting it with a finite Stoke's shift into the confined optical modes of, e.g. a transparent slab, where it is trapped by total internal reflection and absorbed by photovoltaic cells attached to the edges. The limiting concentration ratio for an LSC follows from thermodynamic considerations and is exponential in the Stoke's shift according to CRlim≈(eem3/eabs3)exp [(eabs−eem)/kbT], where eabs and eem are the absorbed and emitted photon energies, respectively. This theoretical maximum exceeds the equivalent of 100 suns for most emitters employed in LSCs to date, yet the value realized in practice is more than an order of magnitude lower, typically in the range 2<CR<10, which remains too low to provide any economic benefit in reducing the cost of photovoltaic power. The following provides a new approach to LSC optical design that enables a doubling or more in CR for any type of emitter, thereby improving the prospect of low-cost, high-performance luminescent concentration.